Patent EA-002531-B1 (EA)
Title: Feed processing for improved alumina process performance
Inventor: HOLLITT MICHAEL
IPC: C01F7/0613
Filed: 1999-08-17
Abstract: 1. A process for the treatment of a Bayer process feedstock prior to transferring the treated feedstock to a Bayer process for extracting refined alumina from the treated feedstock, which treatment process includes the following steps: (a) heating the Bayer process feedstock to a temperature of 400 degree C to 650 degree C by direct contact with combustion gases, and (b) cooling the heated feedstock to a temperature at which it can be handled and fed to the Bayer process, and which treatment process is characterised by controlling the contact time of the feedstock at temperatures in the above range in step (a) to ensure decomposition of trihydrate alumina and monohydrate alumina present, by dehydration, while not: (i) substantially forming monohydrate alumina from trihydrate alumina, or (ii) reducing residual bound water to the extent that extraction is adversely affected, or (iii) allowing sufficient time for contact with water vapour that slower water vapour dependent loss of extractability is experienced. 2. The treatment process defined in claim 1 wherein the contact time in step (a) is less than five minutes. 3. The treatment process defined in claim 1 wherein the contact time in step (a) is less than one minute. 4. The treatment process defined in claim 1 wherein the contact time in step (a) is less than 10 seconds. 5. The treatment process defined in any one of the preceding claims wherein the average particle size of the feedstock supplied to step (a) is relatively fine to ensure that particle shells are not overheated or otherwise affected by water vapour. 6. The treatment process defined in any one of the preceding claims wherein the particle size distribution of the feedstock supplied to step (a) is narrow so that very fine particles are not overheated or otherwise affected by water vapour while the thermal treatment of the cores of coarser particles is completed. 7. The treatment process defined in any one of the preceding claims includes milling the feedstock prior to supplying the feedstock to step (a). 8. The treatment process defined in any one of the preceding claims wherein the feedstock supplied to step (a) contains no more than 5% by weight of material which would be retained on a 5mm aperture. 9. The treatment process defined in any one of the preceding claims wherein the feedstock supplied to step (a) contains no more than 5% by weight of material which would be retained on a 2mm aperture. 10. The treatment process defined in any one of the preceding claims wherein the feedstock supplied to step (a) contains no more than 5% by weight of material which would be retained on a 1mm aperture. 11. The treatment process defined in any one of the <DP=3preceding claims wherein the feedstock supplied to step (a) contains no more than 30% by weight of a material which would pass a 20 micron aperture. 12. The treatment process defined in any one of the preceding claims wherein the feedstock contains no more than 20% by weight of material which would pass a 20 micron aperture. 13. The treatment process defined in any one of the preceding claims wherein the feedstock contains no more than 10% by weight of material which would pass a 20 micron aperture. 14. The treatment process defined in any one of the preceding claims includes preheating the feedstock prior to supplying the feedstock to step (a). 15. The treatment process defined in any one of the preceding claims includes dividing the feedstock on the basis of temperature sensitivity or water vapour sensitivity and selectively introducing the divided feedstock streams into step (a) having regard to the sensitivity or sensitivities. 16. The treatment process defined in claim 15 includes introducing a trihydrate alumina fraction of the feedstock into step (a) where the combustion gases are sufficiently cool so that there is reduced potential for loss of extractability from use of hotter gases to decompose the alumina trihydrate in this fraction. 17. The treatment process defined in claim 15 includes introducing a fine fraction of the feedstock into step (a) where the combustion gases are sufficiently cool so that there is reduced potential for loss of extractability from use of hotter gases to decompose the fines in this fraction. 18. The treatment process defined in any one of the preceding claims includes producing the combustion gas used in step (a) in excess air and supplying to step (a) the combustion gas and air. 19. The treatment process defined in any one of claims 1 to 17 wherein step (a) includes heating the feedstock to the temperature of 400C to 650C by direct contact with combustion gases and air. 20. A Bayer process for extracting refined alumina from a Bayer process feedstock, which Bayer process includes a treatment process for treating the feedstock prior to extracting refined alumina from treated feedstock, which treatment process includes the following steps: (a) heating the Bayer process feedstock to a temperature of 400C to 650C by direct contact with combustion gases, and (b) cooling the heated feedstock to a temperature at which it can be handled and fed to the Bayer process, and which treatment process is characterised by controlling the contact time of the feedstock at temperatures in the above range in step (a) to ensure decomposition of trihydrate alumina and monohydrate alumina present, by dehydration, while not: (i) substantially forming monohydrate alumina from trihydrate alumina, or (ii) reducing residual bound water to the extent that extraction is adversely affected, or <DP=5 (iii) allowing sufficient time for contact with water vapour that slower water vapour dependent loss of extractability is experienced. 21. The Bayer process defined in claim 20 wherein the contact time in step (a) is less than five minutes. 22. The Bayer process defined in claim 20 wherein the contact time in step (a) is less than one minute. 23. The Bayer process defined in claim 20 wherein the contact time in step (a) is less than 10 seconds. 24. The Bayer process defined in any one of claims 20 to 23 Wherein the average particle size of the feedstock supplied to step (a) is relatively fine to ensure that particle shells are not overheated or otherwise affected by water vapour. 25. The Bayer process defined in any one of claims 20 to 24 wherein the particle size distribution of the feedstock supplied to step (a) is narrow so that very fine particles are not overheated or otherwise affected by water vapour while the thermal treatment of the cores of coarser particles is completed. 26. The Bayer process defined in any one of claims 21 to 25 Wherein the treatment process includes milling the feedstock prior to supplying the feedstock to step (a). 27. The Bayer process defined in any one of claims 21 to 26 wherein the feedstock supplied to step (a) contains no more than 5% by weight of material which would be retained on a 5mm aperture. 28. The Bayer process defined in any one of claims 21 wherein the feedstock supplied to step (a) contains no more than 5% by weight of material which would be retained on a 2mm aperture. 29. The Bayer process defined in any one of claims 21 to 28 wherein the feedstock supplied to step (a) contains no more than 5% by weight of material which would be retained on a 1mm aperture. 30. The Bayer process defined in any one of claims 21 to 29 wherein the feedstock supplied to step (a) contains no more than 30% by weight of a material which would pass a 20 micron aperture. 31. The Bayer process defined in any one of claims 21 to 30 wherein the feedstock contains no more than 20% by weight of material which would pass a 20 micron aperture. 32. The Bayer process defined in any one of claims 21 to 31 wherein the feedstock contains no more than 10% by weight of material which would pass a 20 micron aperture. 33. The Bayer process defined in any one of claims 21 to 32 wherein the treatment process includes preheating the feedstock prior to supplying the feedstock to step (a). 34. The Bayer process defined in any one of claims 21 to 33 includes dividing the feedstock on the basis of temperature sensitivity or water vapour sensitivity and selectively introducing the divided feedstock streams into step (a) having regard to the sensitivity or sensitivities. 35. The Bayer process defined in claim 34 includes introducing a trihydrate alumina fraction of the feedstock into step (a) where the combustion gases are sufficiently cool so that there is reduced potential for loss of extractability from use of hotter gases to decompose the alumina trihydrate in this fraction. 36. The Bayer process defined in claim 34 includes introducing a fine fraction of the feedstock into step (a) where the combustion gases are sufficiently cool so that there is reduced potential for loss of extractability from use of hotter gases to decompose the fines in this fraction. 37. The Bayer process defined in any one of claims 21 to 36 wherein the treatment process includes producing the combustion gas used in step (a) in excess air and supplying to step (a) the combustion gas and air. 38. The Bayer process defined in any one of claims 21 to 37 wherein step (a) includes heating the feedstock to the temperature of 400 degree C to 650 degree C by direct contact with combustion gases and air.